I cant say i've had the pleasure of using the Platoon Lance so i dont know about the TP bonuses that it gives(apparently).
But i will say this. I used the Spear+1 until the the level 30 Royal Squire's halberd which has a dmg of 37 instead of 35. SO i do suggest you upgrade to that, even if you've been using the platoon lance because the halberd has a spear delay, rather than the extended lance delay.
Only having experience with the Spear+1 and not the Platoon Lance i must say that i would think the Spear+1 to be the superior choice from level 24 in. With 2 less damage but a much shorter delay, i would think that you would average out to cause more damage with the Spear+1. Though i could be wrong cause i havent used the platoon lance.
The biggest deciding factor on what to use will come down to availability and gil. Unless you want to have a crack at BCNM 20 for the lance or pay 80k (the price when i could've used it on Siren server) then you are not going to be wanting the platoon lance (unless of course you like to have the best of everything for your level then you'd use it until 30). For me i picked up the Spear+1 of our ls leader for only 10k. Now i did buy it several levels b4 i needed it but i think that if you are a Dragoon and you have a limited list of weapons to choose from, then decided 5 levels early what you want and keep an eye out on the AH. it can save you alot of time.
If you are on the Siren server send me a tell anytime. Especially if you are a Dragoon or Galka. There seem to be too few of each in my world.
P.S. If someone who has had experience with both the platoon lance and the Spear+1 could post up there expereinces with which is better that would be great.